The invention relates to a circuit arrangement for teleprinter PBX exchanges, in which a switch-over device serves to connect at least a part of the PBX lines directly to the main connection lines of the network when the PBX exchange is switched off, and serves to reconnect the main connection lines and the PBX lines separately and to the PBX exchange when the said PBX exchange is switched on, this in order to prevent impermissible connections occurring during the switching off and on of the PBX exchange.
As is known, the use of a teleprinter PBX exchange offers the possibility of providing a relatively large number of subscriber stations with access to the switching network via a comparatively smaller number of exchange lines. Normally each connection establishment and each exchange of data takes place via the PBX exchange. If the PBX exchange breaks down, the PBX subscribers no longer have the possibility of establishing connections, receiving data or transmitting data. For this reason, it has already been proposed to lead at least a few of the PBX lines across a switch-over device within the PBX exchange, in which device, in the event of the breakdown of the PBX exchange, these PBX lines are directly connected to the lines of the network, which in the following will be referred to as main connection lines. Therefore, even in the event of the breakdown of the PBX exchange, these links, which are referred to as protected PBX lines, possess access via the main connection lines to a superordinate exchange, via which they can both establish connections and themselves be reached. When the PBX exchange is ready for operation again and thus, for example, the disturbance has been eliminated, the connection of the protected PBX lines to the main connection lines is discontinued and all connections again run across the PBX exchange. However, this gives rise to problems inasmuch as with every switch-over, i.e., each time the protected PBX lines are connected to the main connection lines and each time this connection is cut off, there is no assurance that all the lines will be in the rest state. This can lead to the establishment of impermissible connections following a switch-over via the lines connected to the PBX exchange.